[PATCH][RFC] Lightweight user-level semaphores

[Matthew Kirkwood]

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Linus,

The patch below implements some of your design for a really
quick user-level locking primitive, as explained here:

    http://lwn.net/2001/0419/a/lt-semaphores.php3

There's a user-level API and a couple of test programs in
the attached tarball.  I haven't bothered wih the vital
security hash/signature thing yet.

It all seems to work (i686 UP and SMP), but isn't without
issues:

 * It leaks.  How were you going to refcount the kernel
   portions?  Could they be attached to the VM mapping?
   Would a lockfs be too expensive?

 * It doesn't have a timeout.  Is there something like a
   down_timeout() available?

 * I don't do the:

	if (kfs->user_address != fs)
		goto bad_sem;

   because it doesn't seem to add anything, and prevents
   putting these locks in a non-fixed file or SysV SHM
   map.

   Is that a problem?

Any comments?

Matthew.

diff -ruN linux-2.4.17/arch/i386/kernel/entry.S linux-2.4.17-usersem/arch/i386/kernel/entry.S
--- linux-2.4.17/arch/i386/kernel/entry.S	Sat Jan  5 14:00:37 2002
+++ linux-2.4.17-usersem/arch/i386/kernel/entry.S	Sun Jan  6 13:52:50 2002
@@ -622,6 +622,9 @@
 	.long SYMBOL_NAME(sys_ni_syscall)	/* Reserved for Security */
 	.long SYMBOL_NAME(sys_gettid)
 	.long SYMBOL_NAME(sys_readahead)	/* 225 */
+	.long SYMBOL_NAME(sys_FS_create)
+	.long SYMBOL_NAME(sys_FS_down)
+	.long SYMBOL_NAME(sys_FS_up)

 	.rept NR_syscalls-(.-sys_call_table)/4
 		.long SYMBOL_NAME(sys_ni_syscall)
diff -ruN linux-2.4.17/include/asm-i386/unistd.h linux-2.4.17-usersem/include/asm-i386/unistd.h
--- linux-2.4.17/include/asm-i386/unistd.h	Wed Oct 17 18:03:03 2001
+++ linux-2.4.17-usersem/include/asm-i386/unistd.h	Sun Jan  6 13:49:54 2002
@@ -230,6 +230,9 @@
 #define __NR_security		223	/* syscall for security modules */
 #define __NR_gettid		224
 #define __NR_readahead		225
+#define	__NR_FS_create		226
+#define	__NR_FS_down		227
+#define	__NR_FS_up		228

 /* user-visible error numbers are in the range -1 - -124: see <asm-i386/errno.h> */

diff -ruN linux-2.4.17/include/linux/usersem.h linux-2.4.17-usersem/include/linux/usersem.h
--- linux-2.4.17/include/linux/usersem.h	Thu Jan  1 01:00:00 1970
+++ linux-2.4.17-usersem/include/linux/usersem.h	Sun Jan  6 17:50:04 2002
@@ -0,0 +1,9 @@
+#ifndef __LINUX_USERSEM_H
+#define __LINUX_USERSEM_H
+
+struct fast_sem {
+	int count;
+	void *__opaque_ksem;
+};
+
+#endif /* __LINUX_USERSEM_H */
diff -ruN linux-2.4.17/kernel/Makefile linux-2.4.17-usersem/kernel/Makefile
--- linux-2.4.17/kernel/Makefile	Mon Sep 17 05:22:40 2001
+++ linux-2.4.17-usersem/kernel/Makefile	Sat Jan  5 14:36:39 2002
@@ -9,12 +9,12 @@

 O_TARGET := kernel.o

-export-objs = signal.o sys.o kmod.o context.o ksyms.o pm.o exec_domain.o printk.o
+export-objs = signal.o sys.o kmod.o context.o ksyms.o pm.o exec_domain.o printk.o usersem.o

 obj-y     = sched.o dma.o fork.o exec_domain.o panic.o printk.o \
 	    module.o exit.o itimer.o info.o time.o softirq.o resource.o \
 	    sysctl.o acct.o capability.o ptrace.o timer.o user.o \
-	    signal.o sys.o kmod.o context.o
+	    signal.o sys.o kmod.o context.o usersem.o

 obj-$(CONFIG_UID16) += uid16.o
 obj-$(CONFIG_MODULES) += ksyms.o
diff -ruN linux-2.4.17/kernel/usersem.c linux-2.4.17-usersem/kernel/usersem.c
--- linux-2.4.17/kernel/usersem.c	Thu Jan  1 01:00:00 1970
+++ linux-2.4.17-usersem/kernel/usersem.c	Sun Jan  6 17:50:20 2002
@@ -0,0 +1,82 @@
+/*
+ * Lightweight user-level semaphores
+ */
+
+#include <linux/slab.h>
+#include <linux/usersem.h>
+
+#include <linux/spinlock.h>
+#include <asm/semaphore.h>
+#include <asm/errno.h>
+
+/* Don't be messin' with my van^H^H^Hopaque data, sucka */
+#define	FS_SIG_MAGIC	0xf001f001
+
+struct ksem {
+	unsigned magic;
+	spinlock_t spin;
+	struct semaphore sem;
+};
+
+
+/* XXX - _from_user */
+static inline struct ksem *get_ksem(struct fast_sem *s)
+{
+	struct ksem *r = (struct ksem*)s->__opaque_ksem;
+	if(!r) return NULL;
+	if(r->magic != FS_SIG_MAGIC) return NULL;
+	return r;
+}
+
+/* XXX - _to_user */
+static inline int store_ksem(struct fast_sem *s, struct ksem *k)
+{
+	s->count = 0;
+	s->__opaque_ksem = (void*)k;
+	return 0;
+}
+
+static struct ksem *ksem_new(void)
+{
+	struct ksem *s;
+	s = kmalloc(sizeof(*s), GFP_KERNEL);
+	s->magic = FS_SIG_MAGIC;
+	spin_lock_init(&s->spin);
+	sema_init(&s->sem, 1);
+	return s;
+}
+
+
+asmlinkage long sys_FS_create(struct fast_sem *sem)
+{
+	struct ksem *ksem;
+	if(!(ksem = ksem_new()))
+		return -ENOMEM;
+	if(store_ksem(sem, ksem)) {
+		//ksem_free(ksem);
+		return -EFAULT;
+	}
+	return 0;
+}
+
+
+asmlinkage long sys_FS_down(struct fast_sem *sem)
+{
+	struct ksem *ksem;
+
+	if(!(ksem = get_ksem(sem)))
+		return -ENOENT;
+
+	down_interruptible(&ksem->sem);
+	return 0;
+}
+
+asmlinkage long sys_FS_up(struct fast_sem *sem)
+{
+	struct ksem *ksem;
+	if(!(ksem = get_ksem(sem)))
+		return -ENOENT;
+
+	up(&ksem->sem);
+	return 0;
+}

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